Electrowinning of copper

ABSTRACT

A novel electrode is provided which is particularly useful as an anode in an electrowinning system for the electrodeposition of copper from acid solution. This novel electrode is an alloy of lead and bismuth containing from 95 to 70 parts by weight lead with corresponding proportions of 5 to 30 parts by weight bismuth. The use of this electrode as an anode in conjunction with conventional cathodes in an electrowinning process for copper results in cathode deposited copper having a purity of at least 99.99 percent, and a lead content of no greater than 20 parts per million.

United States Patent [191.

Fountain et al.

[ ELECTROWINNING OF COPPER [73] Assigneel- Capital Wire and Cable Company,

Plano, Tex.

[22] Filed: Feb. 1, 1972 [21] Appl. No.1 222,644

[52] 0.5. CI. 204/108, 204/293 [51] lnt. C1 C22d 1/16, BOlk 3/06 [58] Field of Search 204/108, 293, 106-107 [5 6] References Cited UNITED STATES PATENTS 2,666,029 1/1954 DeQuasie et al. 204/293 2,356,897 8/1944 Stack 204/293 2,321,796 6/1943 Butler 204/293 1,851,219 3/1932 Tainton..... 204/293 1,491,944 4/1924 Burwell 204/293 [1 1, 3,755,112 Aug. 28,1973

1,034,711 8/1912 Ives ..204/93 OTHER PUBLICATIONS Periodic Table of the Elements I Primary Examiner-John H. Mack 57 ABSTRACT A novel electrode is provided which is particularly useful as an anode in an electrowinning system for the electrodeposition of copper from acid solution. This novel electrode is an alloy of lead and bismuth containing from 95 to 70 parts by weight lead with corresponding proportions of 5 to 30 parts by weight bismuth. The use of this electrode as an anode in conjunction with conventional cathodes in an electrowinning process for copper results in cathode deposited copper having a purity of at least 99.99 percent, and' a lead content of no greater than 20 parts per million.

7 Claims, No Drawings ELECTROWINNING OF COPPER This invention relates to electrodes and electrolytic methods. In another aspect, this invention relates to an improved method of producing electrolytic copper with a novel anode of a lead-bismuth alloy.

Even though the worldwide demand for copper is continually increasing and relatively extensive copper production facilities are currently in operation, conventional copper refining and extracting techniques leave much to be desired. Such conventional methods are either expensive, time-consuming, and/or incapable of producing an extremely high quality copper product.

Generally, conventional electrowinning processes for copper utilize lead anodes such as lead-antimony alloy anodes with cathodes made of a suitable material such as copper. While these conventional electrowinning processes produce relatively pure copper, sufficient impurities are contained within the electrolytically deposited copper to prevent such copper from being used in a conventional continuouscasting process for the production of copper rod.

A typical continuous casting process is the Properzi Process and comprises delivering molten metal to a peripheral groove on a continuous rotating casting wheel. An endless metal band encircles a portion of the casting wheel (generally about one-half of the periphery of the wheel), such that a continuous casting mold is defined by the groove in the casting wheel and the overlaying metal band which covers that portion of the wheel. Molten metal delivered to the continuous casting mold is solidified and partially cooled under the metal band as the wheel rotates by utilization of cooling water which, for example, flows through the interior of the casting wheel. A solid cast bar of metal is withdrawn from the continuous casting mold as the metal band parts from the periphery of the casting wheel. The cast metal bar is immediately passed through a rolling mill wherein it is rolled to the proper shape to yield a continuous length of rod having a substantially uniform, relatively small cross-sectional area. A complete disclosure of the Properzi Process is disclosed in U.S. Pat. Nos. 2,659,948; 2,659,949; 2,710,433; 2,789,450 and 2,865,067. Other improvements of this process are disclosed in U.S. Pat. Nos. 3,279,000 and 3,351,126.

Generally, copper having a lead impurity of greater than parts per million which is produced by a conventional electrowinning process cannot be effectively utilized in a Properzi Process. 1f the copper feed to the Properzi Process contains too many impurities, the force applied to strip the cast bar from the casting mold produces stress cracks along the surface of the bar. The subsequent rolling of the bar having these stress cracks results in a series of complete ruptures of the bar as it passes through the rolling mill. Accordingly, copper smelters have produced substantially all the copper for this process. Such smelters have the disadvantages of requiring very large capital expenditures, relatively large operating costs, and the ecological disadvantage of emitting pollutants into the atmosphere.

Therefore, one object of this invention is to provide a novel electrode.

Another object of this invention is to provide a novel electrode which can be used in a conventional copper electrowinning process to produce copper having a lead impurity of no greater than 20 parts per million.

A further object of this invention is to provide a novel electrode which can be utilized to produce cathode dea 2 posited copper which is of sufficient purity to be utilized in a conventional continuous casting process for the production of copper rod.

According to one embodiment of this invention, a novel electrode is provided which consists essentially of a lead-bismuth alloy containing from about 95 to weight percent lead and from about 5 to 30 weight percent bismuth.

According to another embodiment of this invention, the above-described lead-bismuth alloy electrode is utilized as an anode in an electrowinning system for copper which produces copper having a lead impurity of less than 20 parts permillion, and which can be satisfactorily molded in a continuous casting process.

Thus, it has been found that the incorporation of minor effective amounts of bismuth in lead forms an alloy which when used as an anode in electrowinning copper is instrumental in producing copper of unexpected purity. Such anode is relatively stable in conventional aqueous sulfuric acid electrolyte solutions and will not transmit contaminants to the cathodedeposited copper. More specifically, it has been found that the incorporation of from about 5 to 30 weight percent bismuth into lead yields an alloy which can be used to form a superior anode. Based upon such factors as product purity and-life expectancy of the resulting anode, the preferred range of bismuth in the lead is from about 5 to 25 weight percent thereof, and the most preferred range isfrom about 5 to 20 weight percent thereof.

The bismuth-lead alloy which is used in the novel electrode of the subject invention can be made in any conventional manner. For example, the alloy can be made by initially thoroughly blending the two metals in the desired proportions, such as in a furnace to obtain a homogenous blend therebetween, and thereafter casting the alloy into a suitably shaped anode. This is typically done by admixing the two metals in particulate form in a proper ratio and then melting the resulting comminuted blend in a furnace before casting.

The novel electrode of the subject invention can be used in any electrowinning process, but preferably is utilized in the electrowinning of copper from an aqueous acidic electrolyte solution. Typically, copper ions are dissolved in an aqueous sulfuric acid electrolyte solution containing, for example, from about 10 to about 200 grams per liter of H,SO Generally the aqueous sulfuric acid electrolyte will contain dissolved copper in amounts from about 30 to about 55 grams per liter and have a pH from about 0.5 to about 1.0. The cop per-pregnant aqueous sulfuric acid electrolyte is then passed through conventional electrowinning cells containing a large number of alternately disposed cathode and anode sheets hanging generally vertically and disposed normal to the flow of the solution therethrough. Any conventional electrolyte flow pattern can be used in the electrowinning cells containing the novel leadbismuth anodes of the subject invention and generally not affect the efficiency thereof.

Generally, the anode of the subject invention can be utilized in such processes which are operated with an electrolytic current density of from about 10 to about 30 amps per square foot, and a voltage drop per cell (electrode pair) of approximately 2 volts. It has been found that when utilizing the novel anode of the subject invention with a conventional cathode, there is generally from about 10 to 15 percent less current requirement than when using conventional lead or leadantimony alloy anodes. Any conventional cathode can be used with the anode of the subject invention. For example, cathodes can be made of stainless steel, copper or titanium, but preferably they are made of stainless steel.

Thus, the novel anodes of the subject invention can copper cement. The process generally includes dissolving particulate copper in a dilute sulfuric acid solution and then electrolytically extracting the copper from the solution. The copper is initially dissolved in the dilute sulfuric acid electrolyte by mechanically admixing it with the dilute acid which contains a frothing agent.

Alternately, and preferably, the copper can be recovered in an electrowinning system by a multi-step process such as initially leaching copper ions from copper scrap and/or copper cement with aqueous ammonical leaching solution under oxidation conditions. In this method, copper metal is initially oxidized to cupric oxide as follows:

The cupric oxide thereafter reacts with ammonium hydroxide in the aqueous ammonical leaching solution to form an aqueous copper-ammonia complex as follows:

Thereafter, the copper-pregnant ammonical leach solution is passed to an ion exchange zone wherein it is contacted with a liquid organic ion exchange fluid which generally contains an organic ion exchange liquid carried in a suitable water immiscible organic solvent therefor, such as kerosene. A suitable organic ion exchange fluid is a substituted 2-hydroxy benzophenoxime such as disclosed in US Pat. No. 3,428,449 issued Feb. 18, 1969, which is herein incorprated into this specification by reference. The contact between the organic ion exchange fluid and the copper-pregnant ammonical leach solution effects the extraction of the dissolved copper ions from the aqueous ammonical leach solution by the organic ion exchange material. This extraction is set forth below:

Cu(Nll-l 2(OH) 2RH+3H O I H,,OH+H O The organic and aqueous phases are allowed to separate and the copper-pregnant organic ion exchange material is thereafter contacted with the aqueous sulfuric acid electrolyte to be utilized in the electrowinning process. The aqueous sulfuric acid electrolyte generally contains from 100 to 175 grams per liter of sulfuric acid and has a pH of about 1. This contact will effec- -tively strip the majority of the copper from the ion ex change liquid by the following reaction:

H SO +R Cu 2(RH)+CuSO The organic and the aqueous phases are allowed to separate and the aqueous copper-pregnant electrolyte is then passed to the electrowinning cells. Alternately, the above-described ion exchange system can be utilized to transfer copper ions from a dilute acid leach solution to an acidic electrolyte in the manner set forth above, Such dilute acid leach solutions are typically utilized to extract copper from low grade copper-bearing ores.

When the above-described processes are carried out utilizing novel anodes of the subject invention together with conventional cathodes such as stainless steel, cathode-deposited copper is produced which has a purity of 99.992+% (excluding O and a lead impurity of less than 20 parts per million, and specifically about 6 parts per million. Generally, the copper deposited in accordance with this invention contains no single impurity in excess of about, 10 parts per million thereof. This copper can be utilized in the above-described Properzi continuous casting process to make copper rod.

Also, when utilizing the novel anode of the subject invention, it is preferred that the active surface thereof be initially stabilized. The stabilization technique involves the formation of a hard oxide scale on the outer surfaces of the anode. This scale is preferably formed in situ in the electrolyte by a process which includes initially placing the anode in an electrowinning cell containing an electrolyte, for example an electrolyte which contains from 100 to 200 grams per liter of H,SO and from 10 to 50 grams per liter of CuSO,. Next, current is applied to the cell to yield a current density of for example, from 5 to 30 and preferably from 15 to 25 amps per square foot until a soft, easily removable black oxide film is formed on the surfaces of the anode. This process is complete in from about 1- to about 24 hours. The anode is then removed from the electrolyte and the soft film is removed therefrom by any suitable manner, such as wiping, brushing, scraping and the like.

The above procedure is repeated as desired (usually from 5 to about 15 times) until no more soft oxide film forms on the active surface of the anode, and a hard, adherent scale remains. This resulting scale is approximately from 5 to 30 mils in thickness. The resulting stabilized anode can be used in conventional electrowinning systems and will not transmit contaminating particles to the cathodes of such systems.

The following example is given to further illustrate this invention but is not intended to limit the scope thereof.

EXAMPLE Three bismuth-lead alloy electrodes were made containing 20 weight percent bismuth and weight percent lead; l0 weight percent bismuth and weight percent lead; and 5 weight percent bismuth and weight percent lead, respectively. Each of these electrodes was approximately 8 inches X 10 inches X 0.5 inch. Each said. electrode was then placed into an electrolytic cell and paired with a stainless steel cathode. Each electrolytic cell comprised a chamber for receiving electrolyte with the said anode and cathodes disposed therein. An aqueous sulfuric acid electrolyte containing grams per liter of sulfuric acid and containing an average of 30 grams per liter of dissolved copper in the form of copper sulfate was placed in each of the cells. Each of the electrodes was operated under a current density of 25 amps per square foot, and each cell was operated with a voltage drop of about 2 volts. For the initial 24 hours of operation, the anodes were periodically removed from the cell and the resulting soft black oxide film was wiped from the surface of each anode with a cloth. After this treatment period, only a hard adherent scale remained on the active surface of each anode. Each cell was run for 1,000 hours. After this period, each anode was weighted. It was found that each anode had lost about 0.7 percent of its wherein no impurity (including lead) exceeded parts per million. This copper can be readily molded to form copper rod in a continuous casting process. Asa comparison, when the above-described electrowinning process is carried out but utilizing a standard lead anode or a antimony-lead alloy anode (containing 10 weight percent antimony), copper of purity of 99.99 percent is produced but it has a lead impurity of greater than parts per million and is unsuitable for use in a continuous casting process. In addition, the commercially available antimony-lead anodes have a life expectancy of only about 2 years (based upon a degradation of 66 percent of the anode).

While this invention has been described in relation to its preferred embodiments, it is to be understood that various modifications thereof will now be apparent to one skilled in the art upon reading this specification and it is intended to cover such modifications as fall within the scope of the appended claims.

We claim:

1. In a copper electrowinning process wherein an aqueous sulfuric acid electrolyte containing copper ions is introduced into an electrolytic cell having alternating anodes and cathodes, and said cell is thereafter electrolyzed to deposit copper on said cathodes, the improvement comprising: I

utilizing anodes in said cell of an alloy consisting essentially of from about 95 to parts by weight of lead and from about 5 to 30 parts by weight of bismuth.

2. The improved process of claim I wherein said anodes are made of an alloy containing from 95- to parts by weight of lead and from 5 to 25 parts by weight of bismuth.

3. The improved process of claim 1 wherein said an-- odes are made of an alloy containing from 95 to parts by weight of lead and from 5 to 20 parts by weight of bismuth. V

4, The improved process of claim 1 wherein said aqueous sulfuric acid electrolyte contains from 10 to 200 grams per liter of sulfuric acid and from 30 to 55 grams per liter of dissolved copper.

5. The improved process of claim I wherein said cathodes are made of stainless steel.

6. The improved process of claim 1 wherein said anodes carry a hard, adherent oxide scale on the outer surfaces thereof.

ent oxide scale forms. 

2. The improved process of claim 1 wherein said anodes are made of an alloy containing from 95 to 75 parts by weight of lead and from 5 to 25 parts by weight of bismuth.
 3. The improved process of claim 1 wherein said anodes are made of an alloy containing from 95 to 80 parts by weight of lead and from 5 to 20 parts by weight of bismuth.
 4. The improved process of claim 1 wherein said aqueous sulfuric acid electrolyte contains from 10 to 200 grams per liter of sulfuric acid and from 30 to 55 grams per liter of dissolved copper.
 5. The improved process of claim 1 wherein said cathodes are made of stainless steel.
 6. The improved process of claim 1 wherein said anodes carry a hard, adherent oxide scale on the outer surfaces thereof.
 7. The improved process of claim 6 wherein said hard, adherent oxide scale is formed in situ in said electrolyte by the process comprising: a. placing said anodes in said electrolyte cell; b. electrolyzing said cell for a sufficient time to cause a soft, easily removable oxide film to form on the surface of said anodes; c. removing said soft oxide film from said anodes; and d. repeating steps (b) and (c) until said hard, adherent oxide scale forms. 